Download neural tube.

Development of the Nervous
system
EARLY DEVELOPMENT
• By the beginning of the 3rd week of
development, three germ cell layers
become established, ectoderm,
mesoderm and endoderm.
• During the middle of the 3rd week, the
dorsal midline ectoderm undergoes
thickening to form the neural plate.
• The margins of the plate become
elevated, forming neural folds.
• A longitudinal, midline depression, called
the neural groove is formed.
• The 2 neural folds then fuse together, to
form the neural tube.
• Formation of the neural tube is completed
by the middle of the fourth week of
development.
• Neural tube is open
at both ends,
communicating
freely with the
amniotic cavity.
• The cranial opening,
the rostral
neuropore closes at
about 25th day & the
caudal neuropore
closes at about the
27th day
The cranial ⅓ of the
neural tube represent
the future brain
The caudal ⅔
represents the future
spinal cord
Neural Tube Development
Three-vesicles stage (End of 4th Week)
The cranial part of
neural tube dilates to
form brain vesicle. The
caudal part remains
narrow and cylindrical
to give spinal cord
The brain vesicle grows
and gives 3 dilatations
named as:
• Prosencephalon
• Mesencephalon
• Rhombencephalon
Neural Tube Development
Five-vesicles stage (5th Week)
Brain Flexures
By 4th week:
 The neural tube grows rapidly
and bends ventrally with the
head fold, producing two
flexures:
 Midbrain (cephalic) flexure: In
the region of midbrain.
 Cervical flexure: between the
hind brain & the spinal cord.
Later Pontine flexure appears in
the hindbrain, in the opposite
direction of the cephalic &
cervical flexures, resulting in
stretching and thinning of the
roof of the hindbrain.
HB
MB
FB
HB
Differentiation of Forebrain



The prosencephalon or
the forebrain vesicle
differentiates into:
 a median part, the
diencephalon, and
 two lateral cerebral
vesicles or
(telencephalic vesicles.)
The lumen gives the 3rd
ventricle and the 2 lateral
ventricles.
Both cavities
communicate with each
other through a large
interventricular
foramen.(of Monro)
Development of the Cerebrum
The cerebrum develops from the Telencephalon




The cerebral hemispheres
first appear on the day 32 as
a pair of bubble-like
outgrowths of the
Telencephalon.
By 16 weeks, the rapidly
growing hemispheres are
oval in shape and cover the
diencephalon.
The cerebral hemispheres
expand in all directions.
Its medial wall becomes
thin, flat and it is the site of
choroid plexus of the
lateral ventricle.
The wall of the telencephalon is
formed of 3 layers:
• Ependyma: lining the cavity of
the lateral ventricle.
• Mantle layer: Intermediate layer
contains nerve cells (grey matter).
• Marginal layer: Outer layer
contains nerve fibers emerging
from neuroblasts in the mantle
layer (white matter).
As a result of myelination of nerve
fibers, this layer takes on a white
appearance and therefore is called
the white matter of the spinal cord.
As development proceeds
most of the nerve cells
in the mantle layer
migrate to the outer
aspect of the marginal
layer to form the
cerebral cortex. The
remaining cells form the
basal ganglia (corpus
striatum).
By the end of the 3rd month the surfaces of the cerebral
hemispheres are smooth. By 4th month the grey matter grows
faster than the white matter with the result that the cortex
becomes folded into gyri separated by sulci.
The gyri and
sulci effectively
increase the
surface area of
the brain. The
detailed pattern
of gyri & sulci
varies
somewhat from
individual to
individual
• Corpus striatum
appears in 6th week in
the floor of each
cerebral hemisphere.
• The cerebral cortex
differentiates and the
fibers passing to and
from it, pass through
the corpus striatum,
dividing it into
caudate nucleus &
lentiform nucleus.
• This fiber pathway
forms the internal
capsule.
Further expansion of cerebral
hemisphere give C-shape to the
hemisphere itself as well as the
lateral ventricle.
Also the caudate nucleus elongates
and assumes the shape of the lateral
ventricle and remains related to it.
Development of the Cerebral Commissures
• As the cerebral cortex
develops, group of fibers,
the commissures, connect
the corresponding regions
of the cortex in the two
hemispheres.
• These are:
1. Lamina terminalis.
2. Optic chiasma.
3. Anterior commissure.
4. Posterior commissure.
5. Hippocampal
commissure.
6. Habenular
commissure.
7. Corpus callosum.
6
5
4
7
1
3
2
The cortex covering
the surface of the
corpus striatum
grows relatively
slowly, so it is
overgrown by the rest
of the hemisphere
and lies in the depth
of the lateral sulcus.
This is the insula.
Congenital anomalies of the brain
Anencephaly
• Due to failure of
closure of the rostral
neuropore of the
neural tube…
• Failure of complete
development of the
brain , skull and scalp
Microcephaly
• The
circumference of
the head ,
including the
brain is smaller
than normal
Congenital hydrocephalus
• Abnormal
Accumulation of
CSF inside the
brain cavities